The thermal response of aqueous dispersions of a series of synthetic saturated phosphatidylethanolamines was studied by differential scanning calorimetry and by infrared spectroscopy. Dispersions which had not been previously heated above tm, the temperature of the gel to liquid crystalline transition, showed transitions at a higher temperature, tm + h, having a considerably greater enthalpy change. It is demonstrated that the higher temperature transition is due to the simultaneous hydration and acyl chain melting of these saturated phosphatidylethanolamines. This transition has not been observed in the corresponding phosphatidylcholines.